In recent years, a display device provided with a light-detecting sensor (also referred to as a photosensor) has attracted attention. In the display device including a photosensor, a display screen also serves as an input region. A display device having an image capture function is an example of such a display device (see Patent Document 1, for example).
Examples of a semiconductor device including a photosensor are a CCD image sensor and a CMOS image sensor. Such image sensors are used in, for example, electronic appliances like digital still cameras or cellular phones.
In a display device provided with a photosensor, first, light is emitted from the display device. When the light enters a region where an object exists, the light is blocked by the object, and is partly reflected. The light reflected by the object is detected by the photosensor provided in a pixel in the display device. Thus, the existence of the object in the pixel area can be recognized.
In a semiconductor device with a photosensor, light emitted from an object or external light reflected by the object is detected by the photosensor directly or is detected after the light is gathered by an optical lens or the like.